In a semiconductor fabrication process, a substrate polishing apparatus is used to polish a surface of a substrate such as a semiconductor wafer to a flat mirror finish. The substrate polishing apparatus includes a polishing table having a polishing surface thereon. A substrate is pressed against the polishing surface on the polishing table. Then, while a polishing abrasive is supplied onto the polishing surface, the polishing table is rotated to polish the substrate. There has been proposed a substrate measuring device utilizing light as a device for measuring a film on a substrate during a polishing process of the substrate. For example, a film thickness of a substrate is measured by a substrate measuring device, and an end point of a polishing process can be determined based on this measured film thickness.
As this type of substrate measuring device, there has been proposed a stream-type substrate measuring device. For example, Japanese laid-open patent publication No. 2001-235311 discloses a substrate measuring device having a water supply passage provided in a polishing table. The water supply passage has an outlet provided in a polishing surface, and pure water is ejected through the water supply passage to a substrate. Two optical fibers are disposed in a stream. Measurement light is emitted through one of the optical fibers to the substrate, and reflected light is received through the other of the optical fibers from the substrate. Then, a film thickness of the substrate is calculated based on the reflected light.
Thus, the stream-type substrate measuring device can supply water into a through-hole formed in a polishing pad. Accordingly, it is possible to dilute slurry flowing between the polishing table and the substrate into the through-hole, and to remove slurry attached to the substrate. Thus, influence on measurement from the slurry is reduced so as to maintain a required capability of measurement.
However, a large amount of water should be supplied in order to maintain a required capability of measurement. If water for measurement flows from the through-hole onto a surface (polishing surface) of the polishing pad, then slurry is diluted. Dilution of the slurry may have an influence on polishing performance.
Outflow of water will be described in greater detail. When the water supply passage is covered with the substrate, an amount of water outflow is relatively small. However, in most of conventional substrate polishing apparatuses, location of a substrate is positioned away from a rotational center of the polishing table, and the water supply passage is not continuously covered with the substrate. Specifically, according to rotation of the polishing table, there alternately appear periods of time during which the water supply passage is covered with the substrate and periods of time during which the water supply passage is not covered with the substrate. During a period of time during which the water supply passage is not covered with the substrate, the amount of water outflow is increased, thereby diluting the slurry so as to influence polishing performance.
In the aforementioned substrate polishing apparatus, an opening (outlet) is formed in the polishing surface to apply measurement light to the substrate, as described above. The opening formed in the polishing surface should preferably be as small as possible so as to reduce influence on a polishing process. In order to reduce a size of the opening, it is required to reduce a space for receiving the optical fibers. Under these circumstances, optical fibers having a small diameter have been used for film thickness measurement in a substrate polishing apparatus.
However, optical fibers having a small diameter emit and receive a small amount of light. Therefore, there has been desired a substrate polishing apparatus which can improve a ratio of an amount of received measurement light to an amount of emitted light (light-receiving efficiency).